Heterologous (human sperm/hamster egg) in vitro fertilization was used to assess fertilizing capacity of human males. These studies identified certain individuals as subfertile who have routinely determined normal sperm number, motility and morphology. This approach demonstrated that the usual parameters for semen analyses are unsure predictors of male subfertility; only the most extreme semen abnormalities reliably indicate subfertility. Thus, the in vitro penetration of laboratory animal ova by human sperm appears to be an improved technique to assess human fertilization potential. The epidemiologic application of this noninvasive, fast and relatively inexpensive technique is being explored.